From: Christian Surlykke <christian@...>
Date: 2010-08-17T15:55:52+09:00
Subject: Re: Parsing, BNF, TreeTop, GhostWheel, ...

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You may want to look at: http://kanocc.rubyforge.org/

<http://kanocc.rubyforge.org/></shameless plug>


best regards

Christian Surlykke



2010/8/17 Clifford Heath <no@spam.please.net>

> (Sent previously, but it didn't appear. Sorry if you  get this twice).
>
>
> Philipp Kempgen wrote:
>
>> Is this a problem with packrat/PEG parsers in general?
>>
>
> Yes. PEG parsers are greedy (iterate as far as possible and never
> backtrack on that), and also take the first alternative that succeeds,
> without trying subsequent alternatives. It's actually quite useful
> and natural, once you get the hang of it.
>
> Pure PEGs have no lookahead, as Treetop does. It's often necessary
> though, in the light of the above.
>
> Also, your approach will yield the wrong results when you add '-' into
> the mix, because of associativity. "1-2-3" will calculate as 1-(2-3).
> A better way is to use iteration, see
>
> <
> http://github.com/nathansobo/treetop/blob/master/examples/lambda_calculus/arithmetic.treetop
> >
>
> Clifford Heath.
>

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